Using coils and capacitors as components in constructing filters is highly common In the art. As the frequency increases, the effect of the losses in the coils and capacitors starts to significantly influence the properties thereof. In particular the loss due to the internal resistance of the capacitors and the series inductance becomes significant, as well as stray capacitances and the loss resistance of the coils. In order to maintain high performance of the filters at higher frequencies than usually used with lumped elements, it is necessary to use transmission line resonators.
The use of transmission line resonators, in the present context meaning helical, coaxial or strip line resonators, in filters in the frequency range from 50 to 2,000 MHz is well known In the art. With coaxial resonators, these being typically e.g. ceramic and helical resonators, good high-frequency properties are achieved in a small volume. By coupling several resonators in succession, filters generally used in high-frequency technology can be implemented, such filters being needed in widely varying types of radio apparatus. Strip line resonators and microstrip resonators are widely used from about 1 GHz upwards. Typically in the frequency range from 50 MHz to 1.5 GHz helical resonators are used. A helical resonator is typically fabricated from a winding of silver coated copper wire insulated by air from a metal coated housing into which the coil is placed.
The manufacturers of radio apparatus insist on filters being smaller in height, or at least in volume than before, and in spite of that, still having as good a performance as before. A smaller filter volume can be obtained by reducing the number of the resonators in the filter or by implementing the filter using resonators of smaller size. Reducing the number of resonators is often near impossible in practice, and reducing their size means in practice that the resonators are replaced by resonators with electrically poorer properties.
In vehicular and mobile hand phones used in cellular telephone systems, various different filters are used. In the NMT phones used in Scandinavia, a bandwidth of 25 MHz is in use whereas in the E-TACS system used in Great Britain the bandwidth is 33 MHz. Due to the bandwidth and certain technical reasons required by the system, the size of the filters manufactured for E-TACS system is greater than e.g. in filters for NMT and AMPS (the US system). Typically, an Rx filter of an NMT handphone comprises four resonators whereas an equivalent Rx filter of an E-TACS hand phone can be implemented with five resonators. The number of poles required for the other filters of a phone are also much higher in the E-TACS system than in the other systems.
It is also known in the art that with a reduction in size of a resonator there is a corresponding drop in quality factor. This in turn leads to increased bandpass attenuation in the filters, which is undesirable. Since the features of a filter deteriorate along with the reduced quality factor of the resonators when their size is reduced, other methods to substitute them have to be adopted. Therefore, a number of different procedures have been introduced for tuning the frequency of a resonator.
In Finnish patent application No. 913088 a method is disclosed to transfer the specific curve of a ceramic resonator in the frequency plane. Therein, in the electromagnetic field of a resonator, called the main resonator, a second resonator called side resonator is positioned. One end of the side resonator is coupled with a controllable switch to the earth of the circuit or off the earth. When the switch is open, the side resonator serves as a resonator the resonance frequency whereof being at a distance from the resonance frequency of the main resonator, and when the end has been earthed, the resonance of the side resonator approaches the resonance frequency to the main resonator, causing therein a frequency transfer.
Tuning a resonator frequency by positioning a series connection of an inductance and a capacitance diode within the resonator field is described in patent application GB 2,141,880. Therein on the end surface of a dielectric resonator operating in the Giga Hertz range there is placed a closed loop, comprising two inductances and capacitance diodes connecting the inductances. By changing the capacitance of the diodes with an external control voltage, the inductance of the loop changes and this change thereof leads to a change in the resonance frequency of the resonator. The change can be up to 50 MHz.
Another procedure in which a resonance circuit is positioned in the field of the resonator, the resonance frequency whereof being changed by changing the capacitance of the capacitance diode, is disclosed in patent application GB-2 153 598.
In the prior art apparatus, the coupling of a main resonator to a side or secondary resonator is typically by means of electromagnetic coupling. It is difficult to size in advance by means of calculation a frequency tuning circuit, and even minor divergences in the physical location thereof relative to the main resonator affect the properties of the coupling. Such coupling and accurate repeatable tuning thereby requires that the positions of the respective resonators can be accurately repeated. However, this is difficult in practice and leads to variations in the tunability of the resonators and their resonance frequencies, thereby complicating the manufacture of filters made from such resonators since the variations have to be compensated for at some point during manufacture, or even later.